Piezo pickups and magnetic pickups are very different beasts with different output levels (piezo signal is generally 1/10th that of a standard non-hot pickup).

You'd normally want to run a straight piezo into an input with at least 5 Megohm input impedance, and preferably 10 Megohms. With a standard 1 Meg guitar amp input (which in reality is often a less). the bass response of the piezo suffers, so you won't get much bassy thud. And passing the signal through standard guitar tone and volume controls would load the pickup so much that it would be useless.

If you wanted to fit a magnetic pickup in the F-hole cavity (as you know they will produce an audible thud when you hit them), then it won't need to be a bass one. Any cheap guitar pickup with exposed pole pieces should do and you could epoxy it to the underside of the top. (say a tele neck pickup with the cover removed) Obviously you'd want to locate the pickup in the area where you can hit the top from a playing position. I'd suggest using an non wax-potted coil if one is available.

There's not a huge hollow area on a thinline, so any resonance is going to be at a pretty high frequency, so you'll get what you get..

You could fit a simple pre-amp (no visible volume or tone controls), within the control cavity and fit a battery in there as well. You might need to expand the cavity a bit, but you've got a large pickguard to hide it under.

It might be worth experimenting with a small bit of ply and an old guitar pickup and some more ordinary glue for a temporary set up to see how well a guitar pickup would pick up knocks if attached to the underside of the top (it could even be stuck to the pickguard).

Don't forget that any hidden pickup can be trimmed down to only what's necessary to keep everything in place to work, if it makes fitting it easier.